The present invention relates to a power supply circuit to generate an internal power supply voltage from an external power supply voltage, a semiconductor integrated circuit having such on-chip mounted power supply circuit, and an information recording medium to record design data for providing the power supply circuit as a design part in a computer-readable format. For example, the present invention relates to a technology efficiently applied to variably control the internal power supply voltage in response to an operating frequency of the CPU in order to save power consumption of the semiconductor integrated circuit.
When a semiconductor integrated circuit contains a CPU that operates on an internal power supply voltage lower than a power voltage used for an interface between chips, there is known a method of variably controlling the internal power supply voltage in accordance with an operating frequency of the CPU to save power consumption of the semiconductor integrated circuit. Generally, however, an upper bound is placed on the frequency that can operate on the internal power supply voltage supplied to the CPU. The CPU may malfunction if the internal power supply voltage is decreased with a high frequency applied to the CPU. On the other hand, when the CPU operates at a low frequency, it may need to be operated at a high operating frequency by increasing the internal power supply voltage. In such case, there is a problem of when to supply a high frequency to the CPU. Conventionally, there is known a method of using a timer circuit to determine a specified period to be a voltage transition period from the time to start voltage variable control. During this period, the CPU's operating frequency is decreased or the CPU is stopped. After the voltage transition period, an intended operating frequency is supplied to the CPU. According to another prior art, a CPU's replica circuit is created on the same chip and is supplied with an operational clock. An error detection circuit always monitors an operational delay in the CPU replica circuit and selects an internal power supply voltage that does not cause the CPU to malfunction. When the operating frequency is changed, the internal power supply voltage varies with the operating frequency. This is the same as the voltage variable control. The former prior art is exemplified in Japanese Unexamined Patent Publication No. Hei 7 (1995)-287699. The latter prior art is exemplified in “A Dynamic Voltage Scaled Microprocessor System” (Thomas B, et al., ISSCC Digest of Technical Papers, pp. 232–233, February 2000).